[56.01] M4 - A Globular Cluster Hubble Deep Field: The Main Sequence

A 123 orbit exposure with HST in 2 colors (GO-8679) in a
single field of M4 has yielded photometry to \rm V ~
30, \rm I ~28.5. In the proper motion selected
cluster color-magnitude diagram constructed from these and
earlier epoch data, there is an abrupt termination of the
cluster main sequence at about \rm V = 28 (\rm MV =
15.5), fully 2 magnitudes brighter than our limiting
magnitude. This is similar to what is seen among field
Population II subdwarfs of the same metallicity (\rm [m/H]
= -1.3) and suggests that the lowest mass stars capable of
burning hydrogen in their cores at this metallicity have
been detected. Comparison with the current generation of
models suggests that the masses here are near \rm 0.09
M\odot. Of particular interest are 6 objects which have
proper motions appropriate for cluster membership, are
moderately bright, but are undetected in V. The brightest of
these will have \rm (V - I) > 4, redder than any known
metal poor subdwarf. The nature of these objects is
currently unknown but one possibility is that they are
recycled brown dwarfs.